# Period mapping

A mapping which assigns to a point $s$ of the base $S$ of a family $\{ X _{s} \}$ of algebraic varieties over the field $\mathbf C$ of complex numbers the cohomology spaces $H ^{*} (X _{s} )$ of the fibre over this point, provided with a Hodge structure. The Hodge structure thus obtained is considered as a point in the moduli variety of Hodge structures of a given type.

The study of period mappings dates back to the studies of N.H. Abel and C.G.J. Jacobi on integrals of algebraic functions (see Abelian differential). However, until recently, the only period mappings that have been studied were those which correspond to families of curves.

Let $\{ X _{s} \}$ be the family of fibres $X _{s} = f ^{ {\ } -1} (s)$ of a smooth projective morphism $f: \ X \rightarrow S$, where $S$ is a smooth variety. The cohomology spaces $H ^{*} (X _{s} ,\ \mathbf Z ) = V _{ {\mathbf Z}}$ are then provided with a pure polarized Hodge structure, which is defined by a homomorphism of real algebraic groups (cf. Algebraic group) $h: \ \mathbf C ^{*} \rightarrow G _{ {\mathbf R}}$, where $\mathbf C ^{*}$ is the multiplicative group of the field of complex numbers, considered as a real algebraic group, while $$G = \{ {g \in \mathop{\rm GL}\nolimits (V)} : {\psi (gx,\ gv) = \lambda (g) \psi (x,\ y)} \}$$ is the algebraic group of linear transformations of a space $V$ that multiply a non-singular (symmetric or skew-symmetric) bilinear form $\psi$ by a scalar factor; the automorphism $\mathop{\rm Ad}\nolimits \ h(i)$ of $G _{ {\mathbf R}}$ is thus a Cartan involution and $h( \mathbf R ^{*} )$ lies in the centre of $G _{ {\mathbf R}}$. The set $X _{G}$ of homomorphisms $h: \ \mathbf C ^{*} \rightarrow G _{ {\mathbf R}}$ which possess the above properties is naturally provided with the $G _{ {\mathbf R}}$- invariant structure of a homogeneous Kähler manifold and is called a Griffiths variety, while the quotient variety $M _{G} = X _{G} /G _{ {\mathbf Z}}$ is the moduli space of the Hodge structures. The homomorphism $h$ defines the Hodge decomposition $$\mathfrak G _{ {\mathbf C}} = \oplus \mathfrak G ^{p,-p}$$ of the Lie algebra $\mathfrak G$ of the group $G$, where $\mathfrak G ^{p,-p}$ is the subspace in $\mathfrak G _{ {\mathbf C}}$ on which $\mathop{\rm Ad}\nolimits \ h(z)$ operates by multiplication by $\overline{z} {} ^{p} z ^{-p}$. The assignment $h \rightarrow P(h)$, where $P(h)$ is the parabolic subgroup in $G _{ {\mathbf C}}$ with Lie algebra $\oplus _{ {p} \geq 0} \mathfrak G ^{p,-p}$, defines an open dense imbedding of the variety $X _{G}$ into the compact $G _{ {\mathbf C}}$- homogeneous flag manifold $X _{G}$. In the tangent space $$\mathfrak G _{ {\mathbf G}} / \oplus _ {p \geq 0} \mathfrak G ^{p,-p}$$ to $X _{G}$ at the point $h$, the horizontal subspace $$\oplus _ {p \geq -1} \mathfrak G ^{p,-p} / \oplus _ {p \geq 0} \mathfrak G ^{p,-p}$$ is distinguished. A holomorphic mapping into $X _{G}$ or $M _{G}$ is said to be horizontal if the image of its tangential mapping lies in a horizontal subbundle.

It has been established that the period mapping $\Phi : \ S \rightarrow M _{G}$ is horizontal (see , ). The singularities of period mappings are described by the Schmid nilpotent orbit theorem, which, when $S = \overline{S} \setminus \{ 0 \}$ is a curve with a deleted point, asserts that if $z$ is the local coordinate on $S$, $z(0) = 0$, then when $z \rightarrow 0$, $\Phi (z)$ is asymptotically close to $$\mathop{\rm exp}\nolimits \left ( \frac{ \mathop{\rm log}\nolimits \ z}{2 \pi i} N \ \right ) a,$$ where $a \in X _{G}$ and $N \in \mathfrak G _{ {\mathbf Q}}$ is a nilpotent element (see ). The image of the monodromy group $$\Phi _{*} ( \pi _{1} (S,\ s)) \subset G _{ {\mathbf Z}}$$ is semi-simple in every rational representation of the group $G$, while transference of $T$ around a divisor with normal intersections $\overline{S} \setminus S$ in a smooth compactification $\overline{S}$ of the variety $S$ generates quasi-unipotent elements $\Phi _{*} (T) \in G _{ {\mathbf Z}}$( i.e. elements which take roots of unity as eigen values). The importance of the monodromy group is underlined by the rigidity theorem (see , , ): If there are two families of algebraic varieties over $S$, then the relevant period mappings $\Phi _{1}$ and $\Phi _{2}$ from $S$ into $M _{G}$ coincide if and only if $\Phi _{1} (s _{0} ) = \Phi _{2} (s _{0} )$ at a certain point $s _{0} \in S$, and if the homomorphisms $\Phi _{i\star} : \ \pi _{1} (S,\ s _{0} ) \rightarrow G _{ {\mathbf Z}}$, $i = 1,\ 2$, coincide.

Complete results on the structure of the kernel and the image of a period mapping generally relate to the cases of curves and $K3$- surfaces (cf. $K3$- surface). If $\{ X _{s} \}$ is a family of varieties of the type indicated and $\Phi (s) = \Phi (s ^ \prime )$, then $X _{s} \widetilde \rightarrow X _{ {s} ^ \prime }$( Torelli's theorem), while for $K3$- surfaces the maximum possible image of the period mapping coincides with $M _{G}$( see ). In the case of curves, the image of the period mapping has been described partially (Schottky–Yung relations, see , ). The Griffiths conjecture states that a moduli variety permits a partial analytic compactification, i.e. an open imbedding in an analytic space $\overline{M} _{G}$ such that the period mapping $S \rightarrow M _{G}$ can be continued to a holomorphic mapping $\overline{S} \supset S$ for every smooth compactification $\overline{S} \supset S$. Such a compactification is known (1983) only for the case where $X _{G}$ is a symmetric domain .

How to Cite This Entry:
Period mapping. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Period_mapping&oldid=44308
This article was adapted from an original article by A.I. Ovseevich (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article